This invention relates to an acceleration sensor, and more particularly an acceleration sensor and a triaxial acceleration sensor each utilizing piezoelectric ceramics to detect acceleration.
A conventional triaxial acceleration sensor constructed so as to detect acceleration in each of X-axis, Y-axis and Z-axis directions perpendicular to each other by means of a piezoelectric ceramic substrate and a weight is basically constructed and manufactured in such a manner as taught in International Publication WO 93/02342 (PCT/JP92/00882, U.S. Pat. No. 5,365,799), U.S. Pat. No. 5,571,972 and the like.
Also, another acceleration sensor is known in the art which includes a piezoelectric ceramic substrate having acceleration detecting electrodes, output electrodes and wiring patterns for connecting the acceleration detecting electrodes and output electrodes to each other formed on a front surface thereof and a counter electrode pattern opposite to at least the acceleration detecting electrodes formed on a rear surface thereof, a diaphragm joined to the rear surface of the piezoelectric ceramic substrate, and a weight joined to a rear surface of the diaphragm for generating stress in a region of the piezoelectric ceramic substrate on which the acceleration detecting electrodes are arranged depending on acceleration.
Unfortunately, the piezoelectric ceramic substrate is relatively increased in relative dielectric constant, so that the conventional acceleration sensor of course causes an electrostatic capacity to be generated between the wiring patterns and output electrodes and the counter electrode pattern. Spontaneous polarization charges generated between the acceleration detecting electrodes and the counter electrode pattern are accumulated in the thus-generated electrostatic capacity as well. Thus, the conventional acceleration sensor encounters a problem that an acceleration signal in the form of a voltage signal or a current signal outwardly taken out of the output electrodes is reduced in output to a degree. Thus, in the prior art, amplification of the acceleration signal to a desired level is carried out using an amplifier. However, this causes detection accuracy to be deteriorated due to amplification characteristics of the amplifier with an increase in degree of amplification.
Further, the conventional triaxial acceleration sensor, when one of X-axis direction acceleration, Y-axis direction acceleration and Z-axis direction acceleration in the same magnitude is solely applied to the weight, generally causes levels of an X-axis direction acceleration signal generated from an X-axis direction acceleration output electrode X, a Y-axis direction acceleration signal generated from a Y-axis direction acceleration output electrode Y and a Z-axis direction acceleration signal generated from a Z-axis direction acceleration output electrode Z to be incoincident with each other. An increase in difference between the levels leads to a deterioration in detection accuracy. In view of the problem, the prior art is so constructed that an amplification level of the acceleration signal is adjusted by means of an amplifier, to thereby carry out correction for coinciding the levels in the respective directions with each other. Unfortunately, adjustment of the amplification level is highly troublesome, leading to a deterioration in productivity.